This. Go look up "krokodil" if you want to see just how far people will go for a fix.Playing the game rather than being a curmudgeon, the absolute best (and hence most dangerous if people know you have it) things to trade are drugs. Small and light, easily stored/hidden, useful when needed like little else in the modern world.
Pain killers, antibiotics, antihistamines, benzodiazepines, etc.
In collapse they'll be most missed. We don't know how good we have it.
One of my greatest fears of collapse are the percentage of the population on psychotropics. You think people act crazy now? lol
If the world is so messed up that you are considering barter of ammo, then that means people have given up hope on currency. Just think about how many dollars (in whatever denomination) or bullion and coinage is out there...and you mean to tell me that it is *worthless* in this future of yours? This would mean that, not only has the national government completely collapsed, there's no future in sight.
General survival will be the name of the game and you'll find that people form communities. No one will be trading for the first few months/years as people stabilize in this new environment. IF you happen to survive and IF you happen to have neighboring communities whom you don't fight with for resources and IF you choose to trade with them...then the goods of highest value will be the things that can no longer be made, especially the consumable things. Antibiotics. Insulin. Pain management. Then weapons and tools.
To be honest, if things get that bad, trading is so far down the priority list that I'm not spending any time thinking about it in any great detail.